This invention relates to transverse magnetic printing heads and, in particular, to a high resolution magnetic printing head structure having an odd number (greater than one) of high permeability comb members which are offset relative to each other, whereby there is produced in a moving magnetic medium a magnetic image which, when inked by applying toner thereto, produces a visible image having a resolution and quality which is higher than that produced by single and double comb printing heads.
Magnetic printing is an extremely attractive alternative for computer and data communications applications, particularly in comparison to impact printing methods or to ink jet printing methods. Magnetic printing is quiet and rapid, and is extremely flexible in the range of data which may be displayed. Magnetic printers may be employed to produce or reproduce alphanumeric, graphical or pictorial data. In contrast, many high speed impact printers are generally limited to a fixed character set employing a fixed chain or metal or plastic plug.
A detailed description of magnetic printing may be found in U.S. Pat. No. 4,097,871, issued to Berkowitz et al., and which is assigned to the same assignee as the present invention. This patent is expressly incorporated by reference herein. It is to be particularly noted, however, that in a transverse magnetic printing head, the direction of orientation of the magnetic field impressed on the recording medium is perpendicular or transverse to the direction of movement of the medium.
Magnetic printing basically involves the transfer of a dry magnetic ink image from a ferro-magnetic recording medium to a permanent paper medium. To produce on the recording medium the magnetic image which attracts the magnetic ink or toner from a reservoir to the magnetic recording medium in a transverse magnetic printing system, there is provided an elongated magnetic comb structure oriented in a direction transverse or perpendicular to the direction of travel of the magnetic recording medium so that the directions of the recording magnetic fields are also transverse.
The comb itself is composed of material having high magnetic permeability, thereby facilitating concentration of magnetic flux at the tips of the teeth of the comb. Typically, a plurality of current-carrying conductors is threaded through the gaps between the comb teeth, and the current in these conductors controls the level of magnetic flux at the tips of the comb teeth. These current-carrying conductors may be driven independently but, typically and preferable, are configured so as to be driven using coincident current matrix methods, whereby two conductors in the same gap must be carrying current in the same direction in order to generate a magnetic field of sufficient strength to record a magnetic image on the medium.
The region of increased magnetic flux at the comb teeth tips is positioned near the magnetic recording medium to permit the selective magnetization of regions therein which thereafter attract the magnetic ink or toner. In this manner, the currents in the conductors threaded through the teeth of the comb control the patterning of the magnetic ink on the ferro-magnetic recording medium. The ink pattern is later transferred to a paper medium to which the pattern is permanently fixed, and then the magnetic recording medium is subsequently magnetically erased prior to the recording of a new pattern therein.
To achieve high resolution, the magnetic recording must be spaced relatively close together. For example, the teeth of the comb may be spaced apart to form approximately 120 gaps per inch, thereby requiring teeth on 1/120 inch center-to-center spacing. A magnetic printing head structure capable of this high degree of resolution is disclosed, for example, in application Ser. No. 60,921, filed July 26, 1979, refiled as continuation-in-part application Ser. No. 193,398, on Oct. 2, 1980 and issued Jan. 25, 1983 as U.S. Pat. No. 4,370,661 entitled "Easily Assembleable Transverse Magnetic Printing Head", which is assigned to the same assignee as the present invention. This application is hereby incorporated herein by reference. This application describes a magnetic printing head structure in which the teeth of a single, high permeability, magnetic comb structure project through an elongated slot or opening in each of one or more very thin, flexible printed circuit boards. In one embodiment employing two such flexible printed circuit boards, one board carries word line signals, and the other carries bit line signals, thereby enabling the printing head to be operable in a coincident current mode to select comb gaps to produce corresponding recording magnetic fields. Each printed circuit board has printed thereon electrically conductive lines which extend across the slot in the board. These lines typically cross the elongated slots at an angle of approximately 90.degree. relative to the long direction of the slots. It is these printed lines or wires which carry current to magnetically energize selected gaps in the comb. The resulting magnetic flux bridging each selected gap acts to magnetize, in the "transverse" direction, a small portion of the magnetic medium. Because of the extremely small size of both the comb gaps and the electrical conductors which must be threaded through each gap, the resolution of such a magnetic printing head is limited.
Nonetheless, higher print resolutions are desirable to increase the readability and appearance of the printed data. Resolutions of approximately 180 or 360 dots per inch appear to be required for print quality comparable to that of typewritten or conventionally printed material. In addition to these requirements, it is also highly desirable that the printing head be driven in a coincident current matrix fashion. The economies of such a gap selection method are best appreciated by an example. If a 14 inch paper width is employed, then even at a relatively low resolution of 120 gaps per inch, there is required a total of 1,680 gaps. If each gap were to be driven individually, then 1,680 lines would have to be terminated to the electronics, and a suitable edge connector would have to be provided. This problem is even more significant at higher print resolutions. However, by employing coincident current techniques and digit and word line patterns, as specifically described in application Ser. No. 60,921, then a pair of printed circuit boards may be employed, with each circuit board requiring connections for approximately the square root of 1680 circuits.
Another high resolution comb-type magnetic head is disclosed in application Ser. No. 69,233 filed Aug. 27, 1979, refiled as continuation-in-part application Ser. No. 227,863, on Jan. 23, 1981 and issued July 27, 1982 as U.S. Pat. No. 4,342,039 entitled "High Resolution Magnetic Printing Head", and this application is also expressly incorporated herein by reference. This application is also assigned to the same assignee as the present invention, and discloses a high resolution printing head comprising two comb sub-assemblies. Each sub-assembly comprises a high permeability magnetic comb structure disposed through aligned slots in a pair of overlaid, flexible printed circuit boards. The circuit boards have printed thereon electrically conductive lines or leads crossing over the elongated slots at substantially right angles to the length of the slots. The comb structure and conductors are electrically insulated. Additionally, because a flexible printed circuit board is employed, each board may be folded at an approximately 90.degree. angle along a fold line substantially following the line of the elongated slot in the board. A second sub-assembly, substantially equivalent to the first sub-assembly, is then adjacently disposed so as to place the respective magnetic comb structures as close together as possible so that all of the comb teeth in both sub-assemblies are oriented in the same direction. However, the comb structures are aligned so that the teeth of one comb in one sub-assembly are disposed opposite gaps between the comb teeth in the other sub-assembly. In this fashion, the print resolution is doubled. The two sub-assemblies are preferably held together by any convenient mechanical structure which is then surrounded in a moldable composition forming an arched surface which is made flush with the tips of the comb teeth, (for example, by grinding) so as to provide a smooth, non-abrading printing head for placement in close contact with the magnetic recording medium. However, unless complex bi-polar electronic switching circuits are used, the resultant accumulative magnetic flux along the adjacent comb teeth produces a relatively less desirable printed image in which the magnetic ink is distributed in visibly separated dots rather than in a substantially solid pattern.